1. Field of the Invention
The present invention relates to a conductivity-modulated semiconductor device and a method of manufacturing the device, and more particularly, to an IGBT (Insulated Gate Bipolar Transistor) having a breakdown voltage of 1200 V or more and also a method of manufacturing the IGBT.
2. Description of the Related Art
An IGBT is a composite-structure transistor which comprises an upper MOSFET structure and a lower bipolar transistor structure. Its basic structure and operation are detailed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 57-120369.
In an ordinary MOSFET, electrons are injected into the drain region only. If the drain region is thick or has a low impurity concentration, the flow of electrons is greatly hindered. The resistance of its drain region is the largest contributor to the on-resistance of the MOSFET. In an IGBT, on the other hand, the conductivity of the drain region is modulated, and the resistance thereof is extremely low. The on-resistance of the IGBT is therefore low despite the large thickness and low impurity concentration of the drain region.
In the IGBT, however, some of the minority carriers (holes) are accumulated in the drain region as excessive minority carriers. As a consequence, the IGBT cannot be turned off until the minority carriers are expelled from the drain region, even when the voltage applied on the gate is reduced to 0 V to close the channel to turn off the IGBT. Moreover, when the IGBT is turned off, electrons would move from the drain region and through the anode region, so holes are inevitably injected into the drain region. This is why the IGBT has a turn-off time which is 10 times or more longer than that of a MOSFET, though the IGBT can pass a current which is about 10 times greater than can be passed by the MOSFET. Hence, if the IGBT is incorporated into in a switching circuit such as an inverter, the circuit cannot acquire a sufficiently high switching frequency. The IGBT can be applied but to a limited use, due to its long turn-off time.
A method of decreasing the turn-off time of an IGBT is disclosed in Baliga et al., "Fast-Switching Insulated Gate Transistors", IEEE Electron Device Letters, Vol. EDL-4, No. 12, December 1983, pp. 452-454. In this method, an electron beam is applied to the drift region of the IGBT, thereby shortening the lifetime of the minority carriers. Other methods are known, in which a radiation such as a neutron beam or gamma rays are applied, or heavy metal such as Au or Pt is diffused, to obtain the same effect of shortened lifetime of the minority carriers accumulated in the drain or anode region. Although these methods can shorten the turn-off time of an IGBT, they reduce the degree of conductivity modulation. Consequently, the IGBT fails to achieve its greatest advantage, i.e., a low on-resistance. In other words, the on-resistance of the IGBT increases, as does the turn-on voltage thereof.
Another method of decreasing the turn-off time of an IGBT is disclosed in G. Miller et al., "A New Concept for a Non-Punch-Through IGBT with MOSFET Like Switching Characteristics", IEEE, PESC 1989, Record Vol. I, Sep. 21, 1989, pp. 21-24. In this method, the impurity concentration of the P.sup.+ anode region of an IGBT is decreased in order to suppress injection of holes from the anode region into the drain region of the IGBT. If the impurity concentration of the P.sup.+ anode region is reduced, however, the contact resistance between the P.sup.+ anode region and the anode electrode, which is made of a metal such as Au, is high and non-uniform. Consequently, the on-resistance of the IGBT is high and non-uniform.
Still another method of decreasing the turn-off time of an IGBT is disclosed in Kuo et al., "Optimization of Epitaxial Layers for Power Bipolar-MOS Transistor", IEEE Electron Device Letters, Vol. EDL-7, No. 9, September 1986, pp. 510-512. This method is to increase the impurity concentration of the N.sup.+ buffer layer of an IGBT. The method does not seem to reduce the turn-off time of the IGBT as much as desired. The existing vapor-phase growth (or CVD) method cannot form a stable buffer layer since it is impossible to reliably control the growth of the layer if the layer has a high impurity concentration. To be specific, the impurity in the N.sup.+ buffer is diffused into the N.sup.- drain region due to the thermal hysteresis during the manufacture of the IGBT. As a result, the N.sup.+ buffer layer will have a high impurity concentration and a large thickness, making it difficult to sufficiently reduce the turn-off time of the IGBT. In addition, to acquire a high voltage of 1200 V or more, an IGBT needs to have an N.sup.- drain region which has a low impurity concentration (about 5.times.10.sup.13 /cm.sup.3) and a large thickness (100 .mu.m or more). The existing vapor-phase growth method, however, cannot reliably form such a drain region.
Another method of decreasing the turn-off time of an IGBT is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-7569. The method is characterized in two respects. First, a double-diffused drain MOS structure is formed in one of the major surfaces of an N.sup.31 semi-conductor substrate having a low impurity concentration. Second, impurity ions are implanted into the other major surface of the substrate, forming a P.sup.+ anode region therein. In the structure thus formed, however, the P.sup.+ anode region has a small diffusion depth, providing a shallow PN junction of only about 1 .mu.m. Being so thin, the PN junction is easily affected by the surface conditions of the substrate. The characteristics of the IGBT, therefore, cannot be stable.